A simple migration script runner that assumes SQL-based migrations, with related downward migrations in a down/ subdirectory, and PostgreSQL as the backing database, with simple access requirements.

migrate.sh

#!/bin/bash

# default to apache/postgres if env is not set up; probably means we're on
# a vagrant VM; same for work dir
user=${PGUSER:-apache}
db=${PGDATABASE:-postgres}
base=${BASEDIR:-/vagrant}

# get the current schema version, or assume we're on an empty DB/version 0
cur_version=$(psql -t -d $db -U $user -c "SELECT schema_version()" \
              2>/dev/null | head -n1 | tr -d ' ')
if [[ "$cur_version" -eq '' ]]; then
    cur_version=0
fi

# depending on up or down migrations, find appropriate files and run them
# in sequence
if [[ "x$1" == 'xdown' ]]; then
    for f in $( find $base/migrations/down -type f -name \*.sql | \
                sort -r ); do
        version=$(head -n1 $f | cut -d\  -f3)
        if [[ $cur_version -ge $version ]]; then
            psql -d $db -U $user -X -1 -v ON_ERROR_STOP=1 -f $f

            cur_version=$(($cur_version - 1))
            cmd='CREATE OR REPLACE FUNCTION schema_version() RETURNS INTEGER '
            cmd="$cmd IMMUTABLE LANGUAGE SQL AS 'SELECT $cur_version'"

            psql -q -d $db -U $user -c "$cmd" 2>/dev/null
        fi
    done
else
    for f in $( find $base/migrations -maxdepth 1 -type f -name \*.sql | \
                sort ); do
        version=$(head -n1 $f | cut -d\  -f3)
        if [[ $cur_version -lt $version ]]; then
            psql -d $db -U $user -X -1 -v ON_ERROR_STOP=1 -f $f

            cur_version=$(($cur_version + 1))
            cmd='CREATE OR REPLACE FUNCTION schema_version() RETURNS INTEGER '
            cmd="$cmd IMMUTABLE LANGUAGE SQL AS 'SELECT $cur_version'"

            psql -q -d $db -U $user -c "$cmd" 2>/dev/null
        fi
    done
fi

# if we've just finished emptying the DB, then drop our magic function too
if [[ $cur_version -eq 0 ]]; then
    psql -q -d $db -U $user -c 'DROP FUNCTION schema_version()' 2>/dev/null
fi